A. Field of the Invention
The present invention relates to an apparatus, system, and method for generating heat energy from combustion, and in particular, from the combustion of biomass materials that have bridging characteristics.
B. Problems in the Art
U.S. Pat. Nos. 5,839,375 and 6,244,196, of which one of the present inventors is an inventor, disclose combustors for bridging biomass fuels. These patents are incorporated by reference herein in their entireties. The combustors utilize the natural bridging characteristics of that type of fuel. Combustion is instigated from inside a mass of the fuel in the middle of the firebox. The surrounding fuel tends to maintain its position as combustion proceeds inside to outside because of its bridging characteristics. The characteristics of the fuel cause the fuel to resist flowing. This includes a resistance to flowing or easily separating, fracturing, or collapsing once initially filled into a container. This includes a resistance to collapse or cave in by gravity, even though a middle portion is missing or consumed. The fuel also acts as an insulator. It substantially envelopes the combustion zone. This holds heat in for higher combustion temperatures and more complete burning. It also keeps the exterior of the combustor relatively cool. As can be appreciated, these combustors can generate temperatures of many hundreds of degrees Fahrenheit (“F”). If such level of heat were conducted or convected to the relatively thin metal skin or wall of such combustors, it would represent a safety hazard to people, animals, and anything combustible.
Not only can these machines be used to incinerate, and thus dispose of, a variety of materials, the heat energy of the combustion is available for a variety of beneficial uses. For example, combustion air is ejected into the interior combustion zone surrounded by the fuel. A heat exchanger, also in that combustion zone, can extract a substantial amount of the heat energy and make it available for use.
The advantages of the combustors of U.S. Pat. Nos. 5,839,375 and 6,244,196 are many. These combustors are non-complex, can be scaled up or down in size, and can be used to effectively combust a variety of materials which otherwise might simply be thrown away or represent a cost to dispose of. And, energy can be extracted for beneficial use. Appropriately operated, emissions can also be minimized, or at least pass most, if not all, emissions regulations.
Additionally, a significant advantage is that the combustor does not require a refractory or other built-in heat insulation, which are expensive and require significant maintenance. They also may make it economically impractical for small to medium size combustors.
However, the inventors have found that there is room for improvement regarding these types of combustors. For example, they tend to be batch mode; not continuous mode combustors. They are filled with the biomass fuel. Combustion burns the material from inside to outside. Once enough material has been consumed, gravity caves in the material from the top. Combustion either extinguishes or loses its efficiency. The machine must be shut down, refilled with a new batch of fuel, and reignited. This requires significant manual labor. It also involves considerable “down time” for the combustor. It is inefficient and costly, for most situations, to have operating personal try to continuously monitor the status of the combustor and, in particular, try to identify when cave in has or will occur.
U.S. Pat. Nos. 5,839,375 and 6,244,196 do disclose features that reduce some manual labor with respect to operation of the combustor disclosed therein. For example, an optional fill auger along the top of the combustion or fire box allows fuel to be moved mechanically, instead of manually, into the combustor when a new batch is needed. See either U.S. Pat. No. 5,839,375 or 6,244,196, FIG. 2 and/or column 7, line 17 and column 5, line 25. However, this requires an inlet into the firebox. This can reduce efficiency of operation because it provides a path out of the combustor in addition to the exhaust path. Also, there is no effective way to know when fuel refilling is required. Therefore, the fill auger in those patents is primarily a mechanical way to fill in another batch of fuel once the predecessor batch has collapsed or been consumed.
Some prior attempts to operate the above-described types of combustors utilize hydraulics for such things as the fill auger or the fans that supply combustion air or heat exchanger air or fluid. However, it has been found that use of hydraulics have inherent issues when applied to such combustors. This has especially been found to be the case when the combustors are used for a variety of different fuel types, particle sizes, moisture levels, and other fuel or operational characteristics. For example, hydraulic fluid can behave differently depending on weather and/or ambient temperature differences or changes. This, in turn, can cause significant variations in operation of hydraulic motors. This makes it difficult to maintain consistent operation with combustors of the type described above, if hydraulics are utilized. Furthermore, many times pressure is not consistent in the fire box. This can lead to inconsistent heat output and pollution. Many hydraulic fluids are petroleum-based. Any leak or spill can represent an environmental and safety issue.
When the fuel collapses in these machines, safety issues also arise. The fire ball is exposed to the sidewall of the fire box and can heat it to dangerous temperatures. Also, the refractory action of the surrounding fuel is diminished or lost, which affects the efficiency of the combustor. Also, this results in lost heat.
Other issues have arisen with the above types of combustors. Typical access doors to allow interior maintenance of such combustors risk exposing the workers to the fire ball. Rocks or other non-combustible debris in the fuel may cause binding clogging of input or output to the combustor. Stack fires have occurred. Inconsistency or loss of pressure inside the firebox can cause smoking, either out of the stack or even through the input into the firebox.
Also there are issues regarding the ability of these particular types of combustors to handle a variety of fuel particle sizes, fuel types, moisture levels, and other fuel characteristics.
There is, therefore, a real need in the art for improvements in at least some aspects of the combustors such as disclosed at U.S. Pat. Nos. 5,839,375 and 6,244,196.